The present invention relates to a dimmable ballast for a high intensity discharge (HID) lamp, for example, a metal halide HID lamp. The design of electronic ballasts for HID lamps needs to take into consideration that HID lamps have different characteristics than other gas discharge lamps, e.g., fluorescent lamps. In particular, HID lamps have higher ignition voltages, typically 3 Kv peak to peak. Fluorescent lamps have ignition voltages of typically 1 Kv peak to peak. HID lamps have no filaments, so there is no need to preheat filaments. Electronic ballasted fluorescent lamps are typically operated at 30-50 kHz. HID lamps have been operated at these frequencies also, but acoustic resonance often occurs which can cause damage to the lamp due to arcing and often even cause the lamp to explode. As a result, HID lamps are typically operated at lower frequencies in the few 100 Hz range to avoid acoustic resonance. At these low frequencies larger full bridge switching circuits are employed to drive the HID lamp with a square wave without resonant output circuits.
In addition, HID lamps in the prior art are typically ignited with a single pulse starter, leading to reliability problems, as the lamps may fail to strike with such single pulse igniters. Further, when HID lamps are hot, the ignition voltage rises to much higher levels, for example, on the order of 25 kilovolts or so.
These characteristics require that the ballasts for such HID lamps have different characteristics than the ballasts for typical fluorescent lamps. In addition, HID lamps have typically only two connections whereas fluorescent lamps typically have four connections with two of the connections being for the lamp filament. An example of a dimming ballast for a fluorescent lamp is shown in U.S. Pat. No. 6,008,593 assigned to the assignee of the present application.